Online

Lutz, Andrew J.

Mechanical Engineering

2015

PhD

The high-enthalpy flow generated by hypersonic vehicles traveling within the Earth's atmosphere inherently delivers an elevated heat flux to the vehicle surface. In addition to conductive heating, the liberated energy generated by various exothermic chemical reactions occurring at the vehicle surface further augment the total heat load. Quantifying the rates at which these reactions take place is imperative and remains a significant challenge as developers attempt to design the next generation of thermal protection systems. This study focused on nitrogen recombination and carbon nitridation, as these reactions are ubiquitous to the most aggressive atmospheric re-entry trajectories in which carbon-based ablative heat shields are conventionally employed. The 30-kW inductively coupled plasma torch located within the Plasma Diagnostics and Test Laboratory at the University of Vermont was used to produce high-enthalpy nitrogen plasma flow, which sufficiently simulated the various in-flight heat flux processes. A combination of optical-based techniques, including spontaneous emission spectroscopy and laser induced fluorescence were utilized to study the free jet and the interaction of the flow with samples constructed from POCO graphite. Emission measurements within the free stream indicated that the nitrogen flow was in non-equilibrium due to the inverse predissociation of ground state nitrogen atoms into the v = 13 vibrational level of the molecular nitrogen electronic B-state. The degree of non-equilibrium was quantified by determining the overpopulation of ground state nitrogen with respect to equilibrium and its effects were considered throughout the analysis. Results obtained through emission spectroscopy and laser induced fluorescence confirmed that the graphite material behaved as a catalytic surface that actively promoted nitrogen recombination. Additionally, the calculated carbon nitridation rate was several orders less efficient, although its effect on the sample surface erosion was evident in the sample mass loss measurements. Subsequently, an independent set of heat flux measurements performed over materials of varying catalycities further supported the data obtained with optical diagnostics. Furthermore, the heat flux results yielded the surface accommodation factor of graphite for the nitrogen recombination rate and indicated that the surface was slightly less than fully-accommodating.